addListener

AddListener is a method used in event-driven programming to register a function, or listener, to be invoked when a specified event occurs. The listener typically receives an event object or data and runs in response to the event. The API commonly accepts the event name or type and a callback function. Many frameworks allow multiple listeners for a single event and invoke them in a defined order when the event is emitted or dispatched. Some environments also provide a once variant that automatically removes the listener after the first invocation, and a separate removeListener or off function to detach listeners.

In Node.js, the EventEmitter class defines addListener as an alias of on; both register a listener for a named event. The listener remains active until it is explicitly removed with removeListener or off. The addition operation is typically side-effectful, and the return value, if any, is usually the emitter itself to facilitate chaining.

In web browsers, the standard approach is addEventListener, which serves the same purpose for DOM events. Some libraries expose an addListener method to provide a uniform API across environments. In all cases, it is important to remove listeners when they are no longer needed to avoid memory leaks, especially in long-running applications or single-page apps.

See also: on, removeListener, off, once.